[Bug]: Scalable backend-dependent service assigned to SSH fleet and never provisioned

[jvstme]

Steps to reproduce

1. Create an elastic fleet and an SSH fleet.

   $ dstack fleet
    NAME           NODES  GPU  SPOT  BACKEND       PRICE  STATUS  CREATED      
    default        0..    -    auto  *             -      active  2 months ago 
    on-prem        1      -    -     ssh           -      active  1 hour ago   
       instance=0         -    -     ssh (remote)  -      idle    1 hour ago

2. Apply this configuration.

   type: service
   name: test-service
   image: nginx
   port: 80
   retry: true
   
   backends: [aws]
   
   replicas: 0..1
   scaling:
     metric: rps
     target: 1

3. Request the service to trigger scaling.

Actual behaviour

The service is assigned to the SSH fleet and never provisioned.

$ dstack event --within-run test-service
[2026-06-05 11:06:50] [👤admin] [run test-service] Run submitted. Status: SUBMITTED
[2026-06-05 11:06:51] [run test-service, gateway main/my-gateway] Service registered in gateway
[2026-06-05 11:06:51] [run test-service] Run status changed SUBMITTED -> PENDING
[2026-06-05 11:07:07] [job test-service-0-0] Job created on new submission. Status: SUBMITTED
[2026-06-05 11:07:07] [run test-service] Run status changed PENDING -> SUBMITTED
[2026-06-05 11:07:11] [job test-service-0-0] Job status changed SUBMITTED -> TERMINATING. Termination reason: FAILED_TO_START_DUE_TO_NO_CAPACITY (Fleet is at capacity)
[2026-06-05 11:07:14] [run test-service] Run status changed SUBMITTED -> PENDING
[2026-06-05 11:07:18] [job test-service-0-0] Job status changed TERMINATING -> FAILED
[2026-06-05 11:07:58] [job test-service-0-0] Job created on new submission. Status: SUBMITTED
[2026-06-05 11:07:58] [run test-service] Run status changed PENDING -> SUBMITTED
[2026-06-05 11:07:58] [job test-service-0-0] Job status changed SUBMITTED -> TERMINATING. Termination reason: FAILED_TO_START_DUE_TO_NO_CAPACITY (Fleet is at capacity)
[2026-06-05 11:08:03] [job test-service-0-0] Job status changed TERMINATING -> FAILED

Expected behaviour

The service is assigned to the elastic fleet and provisioned.

dstack version

0.20.23 (cannot repro on 0.20.22)

Server logs

Additional information

No response